Cardiologists have a variety of tools available for diagnosing and treating heart conditions. For example, cardiologists may use electrocardiograms (ECGs), echocardiograms, and magnet resonance imaging (MRI) to diagnose and assist in the treatment of heart conditions. Each of these techniques uses different technologies to generate different measurements and/or indications of the condition and operation of the heart. Also, each of these techniques has advantages and disadvantages.
An echocardiogram is an ultrasound-based assessment of the heart. In echocardiograms, ultrasound techniques are used to produce two-dimensional slices and three-dimensional images of the heart. Echocardiograms may be invasive or non-invasive. In a non-invasive echocardiogram, the echocardiography transducer (or probe) may be placed on the chest wall (or thorax) of the subject, while images are taken through the chest wall. This non-invasive technique provides an accurate and quick assessment of the overall health of the heart. With this information, a cardiologist can quickly assess a patient's heart valves and degree of heart muscle contraction (using the ejection fraction). In an invasive echocardiogram, a specialized scope containing an echocardiography transducer (TEE probe) may be inserted into the patient's esophagus, while images are taken from there. In addition to creating images of the cardiovascular system, the echocardiogram can also produce measurements of the velocity of blood and cardiac tissue at arbitrary points using pulsed or continuous wave Doppler ultrasound. Echocardiograms are typically expensive, are difficult to administer and read, and require a trained technician to perform the echocardiogram.
An MRI uses magnetic fields to produce accurate images of internal body parts such as the heart. As with echocardiograms, a cardiologist may use an MRI to quickly assess a patient's heart valves and degree of heart muscle contraction. MRI's, however are very expensive. Further, people who have a pacemaker or an implantable cardioverter defibrillator implanted in their body typically are not allowed to have an MRI.
Impedance cardiography (ICG) is a technology that measures thoracic impedance changes, which are related to changes in blood volume in the heart. As such, ICG may be used to used to track volumetric changes in blood flow during the cardiac cycle. In ICG, probes are non-invasively placed near the patient's ribs and neck and an alternating current is transmitted through the patient's chest via the probes. As blood volume and velocity in the heart change within each heart cycle, the ICG measures changes in impedance and calculates a corresponding blood volume and velocity. As such, ICG may be used to measure stroke volume, cardiac output, systemic vascular resistance, velocity index, acceleration index, thoracic fluid content, systolic time ratio, left ventricular ejection time, pre-ejection period, left cardiac work, heart rate, and the like.
An ECG measures electrical potential between various points of the body. The ECG produces a familiar chart of electrical activity with time, as shown in FIG. 1, that represents the electrical activity of the heart. The ECG is non-invasive and relatively inexpensive.
As shown in FIG. 1, the ECG 100 includes a P wave, a PR segment, a QRS complex, and ST segment, and a T wave. The P wave is the electrical signature of the current that causes atrial contraction (both the left and right atria typically contract generally simultaneously). The PR segment connects the P wave and the QRS complex. The QRS complex (including a Q wave, an R wave, and an S wave) corresponds to the current that causes contraction of the left and right ventricles, which is typically much more forceful than that of the atria and involves more muscle mass, thus resulting in a greater ECG deflection, as shown. The Q wave, when present, represents the small horizontal (left to right) current as the action potential travels through the interventricular septum. The R and S waves indicate contraction of the myocardium. The ST segment connects the QRS complex and the T wave. The T wave represents the repolarization of the ventricles.
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interval is usually 0.12 to 0.20 seconds. The duration of the QRS complex is normally less than or equal to 0.10 second. The QT interval is measured from the beginning of the QRS complex to the end of the T wave. A normal QT interval is usually about 0.40 seconds.
Because the ECG provides information about the electrical activity of the heart, ECGs are often used in diagnosing and treating heart patients. Many rules of thumb have been developed for diagnosis based on ECGs. For example, a P wave rate less than 60 may indicate sinus bradycardia while a P wave rate greater than 100 may indicate sinus tachycardia. A PR interval longer than 0.20 seconds may indicate first-degree heart blockage.
As can be seen, there are great deal of cardiographic measurements that can be used in diagnosing and treating heart conditions. One particular heart condition is called interventricular asynchronism which is a condition in which the timing between the contractions of the different chambers of the heart is out of synch, resulting is suboptimal cardio performance. Another condition is called intraventricular asynchronism which is a condition in which the timing of the contractions within a single chamber of the heart is out of synch, resulting in suboptimal cardio performance. Interventricular and intraventricular asynchronism can often be treated with a pacemaker while many other heart conditions do not respond to a pacemaker.
One technique for diagnosing asynchronism relies on measurements from an ECG, specifically, the QRS complex. A QRS complex width of greater than 0.12 seconds duration, associated with an echographic ejection fraction below 35%, has been used to predict the existence of asynchronism. Many patients with a QRS complex width of greater than 0.12 seconds are eventually treated with a pacemaker. Many of these pacemaker patients (about 30%), however, do not actually have asynchronism and thus do not get improvements from the pacemaker. On the other hand, some patients with a QRS complex width less than a 0.12 second duration do suffer from asynchronism and would nonetheless not be implanted with a pacemaker under conventional techniques, and thus a therapeutic benefit is missed. Further, many patients having asynchronism and a pacemaker receive only marginal improvements from the pacemaker. Therefore, there is a need for better techniques for identifying patients who are more likely to respond to a pacemaker and for identifying patients who are likely to benefit from pacemaker adjustment.